It is common for a torsion drive to be employed between an engine and a drive accessory. For example, in automobiles, a belt drive is often used to transfer power from the engine to various accessories, such as a compressor, alternator, generator and numerous pumps. Should any of these accessories experience a catastrophic failure, such as a seized bearing, the accessory will cease functioning and the belt drive will continue to function properly. Although the belt will slip past the non-functioning accessory, possibly causing damage to the belt itself, it is extremely unlikely that damage to other accessories or the engine will occur as a result of the failure of the accessory.
In small aircraft, however, a belt drive is not used to power accessories from an engine. Rather, accessories are driven directly from gears disposed in an accessory case typically located at the rear of the engine. In this design, should a catastrophic failure occur in an accessory, such as a seized bearing as described above or a broken rotor shaft, excessive torques may be experienced throughout the system, including other accessories or even the engine itself. Because all components, including the engine and its accessories, are linked mechanically via the system of gears and shafts, a failure of any one accessory that causes its rotating components to stop, may cause the entire system of the engine and its accessories to seize up. Such extreme torques also can cause damage to those components as well as the gears used to drive them. For example, the seizing of one component mechanically linked to another also may cause that other component to seize or may cause teeth to break off gears. Thus, although mechanical drives are less likely to fail because of a worn belt, they lack the failsafes that belts provide in preventing damage to components in the event of a catastrophic failure of a component of the system.